Oxytocin (OT) – this hormone has two kinds of target cells and 2 effects:
During labor and delivery the target cells are the smooth muscle fibers of the uterus,
which contract in response. (Positive feedback cycle)
After delivery, OT is involved in the ejection (let-down) of milk from the breasts.
target cells are smooth muscle fibers which surround the milk-secreting glands.
Antidiuretic hormone (ADH, vasopressin) – function is to decrease the urine production.
causes the kidneys to reabsorb more of the water passing through and also decreases the activity of the
It causes vasoconstriction (decrease in diameter) of arterioles if present in large amounts,
and is also known as vasopressin because it tends to raise blood pressure.
In case of low water intake or high output, the body becomes dehydrated.
concentration of water in the blood drops below normal.
Osmoreceptors in the hypothalamus detect this
change and activate the neurosecretory cells that produce and release ADH.
In case of high water
intake, water concentration in the blood becomes higher than normal and osmoreceptors detect this and
cause ADH production to slow or stop, so the excess water can be removed by the kidneys.
Other things that can alter ADH production:
ADH secretion increases (urine output decreases) due to pain, stress, trauma and certain drugs
ADH secretion decreses (urine output increases) due to alcohol
If ADH is not secreted the condition is called diabetes insipidus.
It results in
constant production of very large amounts of urine (up to 20 liters/day)
Thyroid gland – located just below the larynx and consists of 2 lateral lobes connected by a mass
called the isthmus that runs over the front of the trachea.
The gland is filled with microscopic spherical
sacs called thyroid follicles.
The walls of the follicles consists of 2 kinds of cells:
Follicular cells extend to the lumen of the follicle and secret the 2 thyroid hormones,
thyroxine (T4) and triiodothyronine (T3).
Parafollicular cells (C cells) – fewer in number and lie between the follicles.
Thyroid hormone – these are normally stored in fairly large quantities in the thyroid, which is
unusual for endocrine glands.
T3 and T4 are the hormones and they are formed by attaching iodine
atoms to an amino acids, tyrosine.
Folicular cells trap iodine from the blood and at the same time form a
glycoprotein called thyroglobulin.
The thyroglobulin is moved into the lumen of the follicle, where it is
Iodine molecules in the colloid react with tyrosine molecules in the colloid to from
They travel in the blood combined with transport proteins.
Thyroid hormones regulate:
Oxygen use and the basal metabolic rate
Growth and development